This invention relates to a flexographic printing press which includes a central impression cylinder or drum with a plurality of color stations and a dryer system used to dry solvent or water based inks. Each color station includes a plate roll and an anilox roll. The anilox roll transfers ink from an ink fountain to the plate roll. The plate roll carries the print image and imprints the image onto a web which is supported by the central impression cylinder.
Currently, high quality flexographic printing using extensible web materials is done on central impression (CI) presses. The flexographic process offers distinct cost advantages and flexibility compared to other types of printing such as gravure. The advantage of using a CI drum over an in-line flexopress is that the web is stable during the printing process. The stability of the web allows for higher quality printing on a variety of substances including extensible films. An example of an extensible film is 0.001 inch polyethylene. A general trend in the printing industry is to save material cost by using thinner webs or films which are ultimately more extensible. Certain segments of the printing industry also use film that characteristically shrinks when heat is applied. An example of this type of film is Cryovac SSD film.
However, there are limitations to the stability of the web, which affect the overall product being produced. It is important to maintain the stability of the web not only while it passes through the prior stations but also during the time that the web is moving through the machine. Disturbances in the web handling process as the web travels from the unwind, through each machine section, and finally to the rewind lead to print registration errors, printed image or repeat variation, and web distortion.
On modern presses, the ink drying and/or curing process causes the greatest amount of damage to the web. Dryers play a vital role in the printing process. The dryer system on a press is used to extract the carrier, either solvent or water, from the ink that is printed on the web. In the ink formulation, the carrier is mixed with the toner, which gives the ink its color, and is used to transport the toner to the printed surface. The most common dryer system uses hot air, sometimes with the assistance of infrared radiation, which impinges the printed web.
“Between color” dryers are located on the impression cylinder between each color station. The purpose of this type of dryer is to dry the ink to a level that another layer of ink can be printed on top of the previous layer. The advantage of the between color dryer is that the web maintains a controlled temperature due to the impression cylinder. The impression cylinder is normally thermally controlled to one degree Fahrenheit by circulating water through the inner shell of the cylinder. Because the cylinder maintains its temperature, the web, through conduction, will also maintain a stable temperature.
Once the last color is printed on the web, the web leaves the central impression cylinder and enters a dryer section. This section is commonly referenced as the tunnel dryer. While the web is passing through the tunnel dryer, it is supported by a number of the guide or idler rolls.
The tunnel dryer acts like an oven. Its purpose is to evaporate the remaining carrier from the printed web. The problem with the tunnel dryer, once the carrier has been removed, is that the heat of the dryer is absorbed into the web. If too much heat is absorbed into the web, the web temperature will exceed the thermal yield point and start distorting. Extensible films characteristically have low thermal yield temperatures. Also, as a general rule the thinner the film, the lower the yield temperature. These factors are making it more difficult to design and use a tunnel dryer in current flexographic presses.
Another significant issue that results in deformation of the web is caused by the tension imparted on the web as the web travels through the machine. As described previously, the web is supported through the tunnel dryer and between machine sections by idler rolls. In most applications, the web is used to rotate the idler rolls. The web path plays a significant role in the amount of tension required in the system. The longer the web path, the greater the amount of idler roll friction the web needs to overcome or drive, which in turn is proportional to the amount of tension imparted on the web. The extensible films described above can withstand minimal tension before permanent distortion occurs.